where \(\mathbf {z}\)  is \(\mathbf {z}_d^\mathcal {D}, \mathbf {z}_t^\mathcal {D}\)  or \((\mathbf {z}_d^\mathcal {D},\mathbf {z}_t^\mathcal {D})\) , \(N\)  denotes the data size and \(M\)  is the mini-batch size.
As discussed in [1]}, such an estimator is biased, but computing it does not require any additional hyperparameters.
